Ail,
I think there is a thread of truth in what you say but that you're heavily overestimating the impact of the effect you describe, and underestimate other factors which may balance out the situation.
In the past, as the number of people who own a system grows there seems to be a dramatic drop off in the number of games bought per system on a monthly basis. I suspect that one of the (untracked) reasons for this is most systems that have had massive userbases in recent times have had hardware defects which caused people to buy new systems, or the hardware had revisions which encouraged people to buy additional systems; consider that the Playstation and PS2 were plagued by the Disc-Read error, the Gameboy had the Gameboy pocket and Gameboy colour, the Gameboy Advance had the SP, and the Nintendo DS had the DS-Lite. We can't say for sure what kind of an overall impact this has had on the attach rate, but it is reasonable to say that the overall lower sales effect is inflated.
At the same time, one of the more interesting effects of the Wii is that it is much more likely to be used by most or all of the people in a household in comparison to the PS3, XBox 360 or previous consoles; even 13 year old hard-core gamer 'Jimmy' who owns an XBox 360 may actually play Wii games if one is released that interests him. Now, if you go from having '1 or 2 people who are dedicated to buying games on a regular basis' to '1 to 2 people who are dedicated to buying games on a regular basis and 2 to 3 people who will occasionaly buy games' it means that there is the potential for greater sales per system than we have seen in the past.
